Magnetic field instabilities in accretion disks

The stability of magnetic fields within the accretion disks is examined, including the effects of magnetic buoyancy, Rayleigh-Taylor, and Kelvin-Helmholtz instabilities. The purpose of the study is to examine the efficacy of turbulent accretion (alpha-)disk models which depend upon 'magnetic viscosity' to provide the dissipation necessary to account for radial infall. Estimates are provided for the time scales of the dominant instabilities in both the gas pressure-dominated and radiation pressure-dominated regions of a thin accretion disk. Constraints are provided on dynamo processes such that 'magnetic viscosity' operates to permit stationary accretion. It is shown that field amplification may take place even in vertically stably stratified regions of a thin disk by the action of 'magnetic buoyancy-driven' convection. It is demonstrated that the turbulent Reynolds stress responsible for accretion is proportional to the gas pressure even in the radiation-dominated portion of the disk.